SE313005B - - Google Patents

Abstract

1,181,678. Inductive signalling. A. J. MINASY. 20 April, 1967 [6 May, 1966], No. 18116/67. Heading H4U. [Also in Division G4] In a system for detecting "protected" articles passing a check point, each article is "protected" by attaching a small resonant circuit, the check point being provided with means for generating an inductive or electromagnetic field the frequency of which is swept beyond the bandwidth of the "protecting" resonant circuit. The check point may have separate transmitting and receiving coils (Fig. 5, not shown) or may have a pair of capacitive electrodes (Fig. 6, not shown) through which the "protected" article must pass or may be associated with the doorway (Fig. 1) having orthogonally mounted coils common to transmitter and receiver. The transmitter (Fig. 4) is preferably a 1 Mc/s. transistor oscillator 14 the frequency of which is modulated by a varactor diode under control of a 500 c/s. R.C. transistor oscillator 24. The receiver includes a diode detector 30 which feeds a carrier-removing filter and a twin T notch filter 34 (which removes the 500 c/s. component) the output of which feeds a transistorized relay-operating circuit 40 actuating an alarm 42. In the absence of a "protected" article from the vicinity of the check point, the small 500 c/s. output of the receiver is stopped by the notch filter but when a "protected" article is brought near the check point, the frequency sweep of the field beyond the band width of the "protecting" resonance circuit causes 1000 c/s. pulses to pass the notch filter and actuate the alarm. The "protecting" resonant circuit may consist of a small ferrite cored inductor shunted by a capacitor with or without capacitive electrodes on opposite sides of the article. Alternatively a printed circuit may be used to protect such articles as clothing or books from shop-lifting.